1. Field of Invention
This invention relates to determining spectra based on non-spectral inputs.
2. Description of Related Art
Automatic on-line color calibration systems can be much more effective with an on-line color measurement system where a spectrophotometer may be mounted in the paper path of the moving copy sheets in the printer, preferably in the output path after fusing or drying, without having to otherwise modify the printer, or interfere with or interrupt normal printing, or the movement of the printed sheets in said paper path, and yet provide accurate color measurements of test color patches printed on the moving sheets as they pass the spectrophotometer. That enables a complete closed loop color control of a printer.
A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. This spectrophotometer desirably provides distinct electric signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges or channels.
Known devices capable of providing distinct electric signals corresponding to the different levels of reflected light received from the respective different illumination wavelength ranges or channels include a grating-based spectrophotometer made by Ocean Optics Inc., LED based sensors marketed by xe2x80x9cColorSavvyxe2x80x9d or Accuracy Microsensor; and other spectrophotometers by Gretag MacBeth (Viptronic), ExColor, and X-Rite (DTP41). However, those devices are believed to have significant cost, measurement time, target displacement errors, and/or other difficulties, for use in real-time printer on-line measurements.
As used herein, unless otherwise specifically indicated, the term xe2x80x9cspectrophotometerxe2x80x9d may encompass a spectrophotometer, calorimeter, and densitometer, as broadly defined herein. The definition or use of such above terms may vary or differ among various scientists and engineers. However, the following is an attempt to provide some simplified clarifications relating and distinguishing the respective terms xe2x80x9cspectrophotometer,xe2x80x9d xe2x80x9ccalorimeter,xe2x80x9d and xe2x80x9cdensitometer,xe2x80x9d as they may be used in the specific context of specification examples of providing components for an on-line color printer color correction system, but not necessarily as claim limitations.
A typical xe2x80x9cspectrophotometerxe2x80x9d measures the reflectance of an illuminated object of interest over many light wavelengths. Typical prior spectrophotometers in this context use 16 or 32 channels measuring from 380 nm to 730 nm or so, to cover the humanly visible color spectra or wavelength range. A typical spectrophotometer gives color information in terms of measured reflectances or transmittances of light, at the different wavelengths of light, from the test surface. (This is to measure more closely to what the human eye would see as a combined image of a broad white light spectra image reflectance, but the spectrophotometer desirably provides distinct electrical signals corresponding to the different levels of reflected light from the respective different illumination wavelength ranges or channels.)
A xe2x80x9ccalorimeterxe2x80x9d normally has three illumination channels, red, green and blue. That is, generally, a xe2x80x9ccalorimeterxe2x80x9d provides its three (red, green and blue or xe2x80x9cRGBxe2x80x9d) values as read by a light sensor or detector receiving reflected light from a color test surface sequentially illuminated with red, green and blue illuminators, such as three different color LEDs or one white light lamp with three different color filters. It may thus be considered different from, or a limited special case of, a xe2x80x9cspectrophotometer,xe2x80x9d in that it provides output color information in the trichromatic quantity known as RGB.
Trichromatic quantities may be used for representing color in three coordinate space through some type of transformation. Other RGB conversions to xe2x80x9cdevice independent color spacexe2x80x9d (i.e., RGB converted to conventional L*a*b*) typically use a color conversion transformation equation or a xe2x80x9clookup tablexe2x80x9d system in a known manner.
A xe2x80x9cdensitometerxe2x80x9d typically has only a single channel, and simply measures the amplitude of light reflectivity from the test surface, such as a developed toner test patch on a photoreceptor, at a selected angle over a range of wavelengths, which may be wide or narrow. A single illumination source, such as an IR LED, a visible LED, or an incandescent lamp, may be used. The output of the densitometer detector is programmed to give the optical density of the sample. A densitometer of this type is basically xe2x80x9ccolor blind.xe2x80x9d For example, a cyan test patch and magenta test patch could have the same optical densities as seen by the densitometer, but, of course, exhibit different colors.
A multiple LED reflectance spectrophotometer, as in the examples of the embodiments herein, may be considered to belong to a special class of spectrophotometers which normally illuminate the target with narrow band or monochromatic light. Others, with wide band illumination sources, can be flashed Xenon lamp spectrophotometers, or incandescent lamp spectrophotometers. A spectrophotometer is normally programmed to give more detailed reflectance values by using more than 3 channel measurements (for example, 10 or more channel measurements), with conversion algorithms. That is in contrast to normal three channel calorimeters, which cannot give accurate, human eye related, reflectance spectra measurements, because they have insufficient measurements for that (only 3 measurements).
It is desirable for a printer color control system to dynamically measure the color of test patches on the printed output media xe2x80x9con linexe2x80x9d, that is, while the media is still in the sheet transport or paper path of a print engine, for real-time and fully automatic printer color correction applications.
For a low cost implementation of the color sensor, a multiple illuminant device is used as the illumination source, and has, for example, 8, 10, 12 or 16 LEDs. Each LED is selected to have a narrow band response curve in the spectral space. Therefore, for example, ten LEDs would correspond to ten measurements in the reflectance curve. The LEDs, or other multiple illuminant based color sensor equivalent, e.g., lasers, are switched on one at a time as, for example, the measured media is passed through a transport of a printer. The reflected light is then detected by a photodetector and the corresponding voltage integrated and normalized with a white tile.
To obtain a smooth curve similar to that of a Gretag spectrophotometer, linear or cubic spline algorithms could be used, which blindly interpolates the data points without the knowledge of the color space. Unfortunately, due to lack of measurements at wavelengths below 430 nm and above 660 nm (due to lack of LEDs at these wavelengths), extrapolation with 10 measurements can lead to errors.
The systems and methods of this invention use the integrated sensor measurements to determine a fully populated reflectance spectra with reflectance values at specific wavelengths, even though some of the light sources may not produce spectral content at the distant ends of the visible spectrum. By using a reconstruction algorithm, based on the spectral characteristics of the illumination source and the color sensing system, the integrated multiple illuminant measurements from a non-fully illuminant populated color sensor are converted into a fully populated spectral curve.
Algorithms according to this invention utilize a reference database that contains training samples that indicate reflectance spectra and their corresponding LED sensor output. A dynamic, Least Squares-based (DLS) spectral reconstruction algorithm is used to reconstruct spectra. The algorithm is xe2x80x9cdynamicxe2x80x9d because it gives greater importance to the data from the training samples in the neighborhood of the color sample under measurement. This is done using linear operators.
These and other objects, advantages and salient features of the invention are described in or apparent from the following description of exemplary embodiments.